1. Technical Field
The present invention relates generally to a sensor network including a plurality of sensor nodes. More particularly, the present invention relates to a method for improving reliability of data transmission when data collected at a sensor node is transmitted to a sink node.
2. Related Art
A related art mobile communication system transfers data between a mobile element and a base station. That is, the mobile element and the base station directly transmit and receive data between each other without passing through other nodes. Meanwhile, a related art sensor network takes advantage of other nodes when transmitting data from a certain node to a sink node. Hereinbelow, the structure of the related art sensor network is described in reference to FIG. 1. The sensor network is constructed with a plurality of sensor nodes including a sink node.
The sink node, which is located in a sensor field, requests the sensor nodes to collect its required data. The data requested by the sink node pertains to temperature or other conditions of the sensor field. The sensor nodes in the sensor field collect the data as requested by the sink node and provide the collected data to the sink node. In this case, the sensor nodes within a certain distance from the sink node transmit their collected data directly to the sink node, and the sensor nodes not within the certain distance from the sink node forward their collected data to another sensor node, which in turn forwards the received data to still another sensor node or the sink node.
When the sensor node forwards its collected data to another sensor node rather than directly to the sink node, the reliability of the forwards data is reduced. Referring to FIG. 1, a node 10 forwards its collected data to a node 9, and the node 9 forwards the received data and its collected data to a node 8. In this manner, the collected data at the node 10 is delivered to a node 20 which is the sink node. The data delivery from the node 10 to the node 20 needs to pass through the node 6 through the node 9. When an error occurs at the data received from the node 6 through the node 9, a method is required to promptly handle the data error.
FIG. 2 illustrates the delivery of the data collected at the node 10 to the node 20 which is the sink node of FIG. 1. The node 20 requests the node 10 to collect and transmit its required data (S200). Typically, the node 20 requests the transmission of the collected data by use of a query message. As the node 10 is not a one-hop node with respect to the node 20, the node 20 sends a query message to a one-hop node 6. The node 6 receiving the query message sends the query message to the node 7 (S202), and the node 7 sends the received query message to the node 8 (S204). The node 8 sends the received query message to the node 9 (S206), and the node 9 sends the received query message to the node 10 (S208).
The node 6 through the node 9 carry out a predefined update process with respect to the received query message, and the updated query message is forwarded to a neighbor node. Upon receiving the query message, the node 10 collects data as requested in the received query message.
The node 10 transmits the collected data to the node 9 (S210), and the node 9 forwards the received data to the node 8 (S212). The node 8 forwards the received data to the node 7 (S214), and the node 7 forwards the received data to the node 6 (S216). The node 6 forwards the received data to the node 20 (S218), and thus the node 20 receives the requested data.
The node 20 receiving the requested data sends an acknowledgement (ACK) message to the node 6 (S220), and the node 6 forwards the received ACK message to the node 7 (S222). The node 7 forwards the received ACK message to the node 9 (S224), and the node 6 forwards the received ACK message to the node 9 (S226). The node 9 forwards the received ACK message to the node 10 (S228), and the node 10 recognizes that the node 20 has received its transmitted data.
As such, a preset time is taken for the node 10 to recognize that the node 20 receives its transmitted data. Generally, a lowest limit of the preset time is the summation of the time required for the node 20 to receive the transmitted data from the node 10 and the time required for the node 10 to receive the ACK message from the node 20. Hence, the node 10 cannot promptly discover a transmission error of its transmitted data.
In addition, if a plurality of sink nodes request the same sensor node to transmit data, load of the sensor network is increased due to the multiple query messages transmitted from the plurality of the sink nodes to a single sensor node.